Subtitles rework + support for dailymotion

I refactored the subtitle support. In this moment is based in a generic InfoExtractor class called SubtitlesIE which makes most of the logic of the subtitle options, then for each website developers must implement only one method to get the subtitles (with the exception of the case of automatic subtitles for youtube which is a second method).
The easiest way to understand how this works is looking at the support for dailymotion that I include with this patch too.
I forgot to tell you that my idea is to make these subtitle not as inheritance in the future but more like decorators, so we can add decorators in sequence to look for subtitles in differents websites like amara.